1. Field of the Invention
The present invention relates to a laser diode driver (hereafter denoted as LD-Driver) that drives a semiconductor laser diode (hereafter denoted as LD), and an optical transmitter providing the LD-Driver.
2. Related Prior Art
An ordinary LD applicable to the optical communication system has a differential resistance of 5 to 10Ω. Such an LD is enclosed within a coaxial CAN package and driven by the LD-Driver by being provided with a driving current thorough a transmission line formed on, for instance, a flexible printed circuit board and a rigid circuit board. Because a distance between the LD-Driver and the LD is a few tens of milli-meters, the driving signal degrades due to the signal reflection between the transmission line and the LD or between the transmission line and the LD-Driver as the transmission rate of the optical communication becomes faster.
Various prior arts have reported the back termination circuit to reduce the back reflection caused by the impedance mismatching between the transmission line and the LD-Driver. However, such back termination circuits disclosed in the prior arts provides only a resistor connected between two outputs of the LD-Driver or between the output of the LD-Driver and the power supply. Such configurations shunt the driving current to be provided to the LD, which increases the power consumption of the LD-Driver. Another back termination circuits temporarily creates a replica signal whose magnitude and phase are comparable to the driving signal. However, the circuit to create the replica signal enlarges the size of the LD-Driver, which also increases the power consumption of the LD-Driver, and makes it hard to lower the power supply voltage.
The back termination circuit according to the present invention suppresses the degradation of the driving signal due to the signal reflection at the outputs of the LD-Driver with less increment of the power consumption.